Isoniazid (INH) is a pro-drug that is activated by catalase peroxidase (KatG) of Mycobacterium tuberculosis (Mtb) to produce isonicotinyol (INacyl) radicals that react with NAD to produce mycolate synthesis inhibitors.
Recently, it has been shown that isoniazid specifically substituted in the acyl position with 13C greatly increase isoniazid activity, through enhanced formation of INH-NAD and NADP adducts after its activation by the bacterial enzyme KatG, with these adducts being the species that are highly toxic to mycobacteria. It is believed that this is due to isotope effects on several steps of the pathway, although effects in the reversibility of acyl radical addition to the NAD, perhaps play the major role. It has been shown that another major TB drug, ethionamide (and its related compound prothionamide), undergoes almost identical activation and adduct formation chemistries (Wang, et al. J. Exp. Med. 204; 73-8 (2007)).
We found that INacyl radicals also decompose to release CO, and so lose the ability to produce INacyl-NAD adducts. Since CO induces dormancy in Mtb, we hypothesized that reducing this decarbonylation by selective 13C substitution (to produce heavy isoniazid, hINH) would increase activity of the resultant drug by maximizing INacyl-NAD formation, and minimizing dormancy induction by CO. Here, we describe how this 13C magnetic isotope effect (MIE) can produce more effective drugs in combination with static magnetic fields greater than the Earth magnetic field.